1. Field of the Invention
The present invention relates to light wave transmission systems and more particularly to a method and apparatus for reducing the influence of polarization mode dispersion in fiber optic transmission lines.
2. Description of the Related Art
Since its invention in the 1970s, optical fiber has been used extensively to carry data signals from one location to another, both terrestrially and undersea. Compared with conventional metallic-based transmission systems, optical fibers offer many advantages, including higher bandwidth, lower cost, lower power consumption, smaller space needs, insensitivity to electromagnetic interference, and greater security. At the same time, however, optical fibers have limitations.
One of the significant limiting factors in high-speed fiber transmissions is the phenomenon known as polarization mode dispersion. Polarization mode dispersion (PMD) arises from the natural birefringence of an optical fiber that occurs when a perfect circular symmetry of the fiber is disrupted. This lack of symmetry may be introduced during fabrication of the fiber or may arise from bending or other physical stress on the fiber. PMD causes a light pulse to broaden during transmission along the fiber and therefore gives rise to distortion and data errors.
As is known in the art, each distributed fiber link of an optical transmission system has two orthogonal polarization modes, or eigen modes, referred to as Principle States of Polarization (PSP). These PSPs are an optical property of the fiber. When a light wave enters the fiber, the wave is resolved into two orthogonally polarized components aligned respectively with these two PSPs. An input light wave of arbitrary polarization may thus be expressed as a sum of components along these PSPs, which may be referred to as the PSP components of the light wave.
The two PSPs of a fiber are degenerate and have identical group delays only when the fiber profile is circularly symmetric and the fiber material is isotropic. Any asymmetry (such as slight ovality) or anisotropy will remove the degeneracy and cause the fiber to become birefringent. Due to this birefringence, the fiber will exhibit different indices of refraction for its two PSPs, which will cause the two PSP components of a light wave to propagate at different speeds down the fiber. As shown in FIG. 1, this difference in propagation velocity will split the light wave, thereby dispersing and distorting the light wave as it travels. Consequently, a single light pulse at the transmitting end may arrive as two light pulses at the receiving end.
The two PSP components of a light wave will have relative amplitudes determined by the polarization of the input light wave. For a given set of PSPs, if the input polarization falls midway between the two PSPs, then the PSP components of the light wave will be equal in amplitude. In that case, the maximum possible PMD (or worst-case PMD) will occur, since the light wave will be split into two equal but separately-propagating components. On the other hand, if the input polarization is aligned with exactly with one of the PSPs, then no PMD will occur (i.e., the best-case PMD), since the entire wave will travel along that one PSP.
The PMD effect can be particularly troublesome for transmission of optical data streams that contain closely spaced symbols, as PMD can cause adjacent symbols to overlap and become indistinguishable. In high-speed optical transmission systems carrying digital data, for instance, PMD can cause adjacent zeros and ones to overlap, thereby introducing bit stream errors. Additionally, since the two PSP components of a light wave travel at different velocities, PMD is especially troublesome in long distance optical transmission systems, since the PSP components of the light wave will continue to disperse over the length of the system.
The PMD effect in most optical transmission systems is also not constant over time or distance, even over individual optical links. Rather, the PMD effect typically varies slowly and unpredictably throughout the system. These variations occur for many reasons. For instance, portions of fiber may be bent or otherwise subject to added stress, which may impact symmetry and therefore alter the PSPs of the fiber. These changes to the PSPs will in turn vary the PMD effect on a light wave of a given polarization. Additionally, the PSPs of a fiber can change with differences in temperature, which may arise from changes in sunlight or ocean currents.
The distorting PMD effect imposes a limitation on the distance that an optical transmission system can competently transmit data. This distance is conventionally referred to as the PMD-limited transmission distance of the system. A transmission system that has a long PMD-limited transmission distance will require fewer repeaters to regularly receive and regenerate the optical waveform. On the other hand, a transmission system that has a short PMD-limited transmission distance will require more closely spaced repeaters, which will increase the complexity and cost of the system.
In an effort to increase this PMD-limited transmission distance, several mechanisms have therefore been proposed for minimizing the PMD effect in optical transmission systems. One technique involves applying a “PMD equalizer,” which uses feedback to change the input polarization of a light signal to be the same as one of the PSPs of the transmission system, or which may alternatively process the output light signal to appropriately combine its PSP components or remove the PSP component that exhibits a higher error rate. Examples of PMD equalizers are discussed in U.S. Pat. No. 5,311,346, entitled “Fiber-optic Transmission Polarization Dependent Distortion Compensation” and issued on May 10, 1994 to AT&T Bell Laboratories; T. Takahashi, T. Imai, M. Aiki, “Automatic compensation techniques for timewise fluctuating polarisation mode dispersion in in-line amplifier systems,” Electronic Letters, Vol. 30, No. 4, 348 (February 1994); J. H. Winters, M. A. Santoro, “Experimental Equalization of Polarization Dispersion,” IEEE Photonic Technology Letters, Vol. 2, No. 8 (August 1980).
Another technique that has been suggested to minimize the effect of PMD and to thereby increase the PMD-limited transmission distance of an optical transmission system is to transmit data in the form of “solitions.” Solitions are single light pulses of a special shape and sufficient amplitude that have been shown to propagate indefinitely along a dispersive fiber without being broadened. The resistance of solitions to PMD is discussed in X. Zhang, M. Karlsson, P. A. Andrekson, K. Bertilsson, “Solition Stability in Optical Fibers With Polarization-Mode Dispersion,” IEEE Photonic Technology Letters, Vol. 10, No. 3, 376–78 (March. 1998).
Although solitions can be used to avoid PMD, however, each solition in a optical data stream needs to be separated from its adjacent neighbors by a certain minimum distance. Absent sufficient spacing between adjacent soltions, solition-interaction will occur, which will pull a solition from its normal position and distort the signal. This spacing requirement, of course, limits the effective bit rate in a solition-based transmission system and consequently renders solitions undesirable for high-speed optical transmission.
Recognizing that PMD will cause errors in an optical data stream, another technique 110 that has been proposed for reducing the PMD effect is to apply forward error correction (FEC).
This technique is described, for instance, in K-P. Ho, C. Lin, “Performance Analysis of Optical Transmission System with Polarization-Mode Dispersion and Forward Error Correction,” IEEE Photonics Technology Letters, Vol. 9, No. 9, 1288–90 (September 1997).
FEC is commonly used to cure a variety of system impairments, such as errors that arise in wireless digital communications, for instance. In general, an FEC encoder receives a group of k information symbols (e.g., bits or bytes) and converts the information into a group of n symbols, where n>k. The group of n symbols output by the FEC encoder is referred to as a codeword or FEC block. In this process, the encoder adds p=n−k redundancy or parity symbols to the input group. At the receiving end, a corresponding FEC decoder may then use this parity information to recover from up to a predetermined maximum number of errors per FEC block. This maximum permissible number of errors may be referred to as the FEC error tolerance.
The effectiveness of FEC coding will thus depend in part on the quality and quantity of parity information transmitted in each FEC block, and in part on the error rate of the transmission channel. If the error rate over the duration of a given FEC block exceeds the FEC error tolerance, then the FEC decoder may be unable to remedy any errors in the FEC block, and the process of FEC encoding may in fact do more harm than good (for instance, as it unnecessarily increases the data rate).
The FEC encoding process works well to recover from individual errors or from at most the FEC error tolerance level of the system. An additional problem arises, however, when burst-errors occur, as burst errors can distort or destroy all or much of the parity information encoded in an FEC block. One method that has been used to overcome from burst errors in both optical and other transmission systems is to interleave the symbols of the FEC codeword so that they experience independent fading. In independent fading, the symbols affected by a burst error typically belong to several different codewords. Therefore, the effect of the burst error can be spread over the message so that it may be possible to recover the data with the original error-correcting code.
In an optical transmission system, the slowly-changing nature of PMD can give rise to errors analogous to a burst error. In particular, it is possible that a light wave of a given polarization may suffer from the worst-case PMD effect for so long at a time that the number of transmission errors arising over a given FEC block will exceed the FEC error tolerance. As a result, just like a conventional burst-error, the FEC decoder may be unable to fully recover from errors in the FEC block without sufficient interleaving.
Unfortunately, however, since the PMD effect can vary so slowly and unpredictably throughout the optical transmission system, PMD-based errors can also at times be analogous to many burst errors in a row. Specifically, it is possible that a steady worst-case PMD effect will cause the error rate to exceed the FEC error tolerance over many interleaved FEC blocks at once. As a result, those skilled in the art have concluded that no practical way exists to interleave an optical signal sufficiently to overcome from these PMD-based burst errors. (See K-P. Ho and C. Lin, at 1288, 1290).
In view of the deficiencies in the existing art, a need therefore exists for a improved system of reducing the influence of PMD in optical transmission channels.